A common problem in operating melters or furnaces for melting mineral materials having high melting points, such as glass, is that the materials within the furnace must be capable of withstanding the effects of two different environments. First, the materials must be able to withstand the environment (i.e., exposure to air) at start up during which time the temperature will gradually increase from room temperature to a high operating temperature, perhaps as high as 3000.degree. F. For various reasons this start up procedure can take up to seven days or more. The second environment that the apparatus experiences is that of continuous operation during which the furnace must function efficiently day in and day out at the high operating temperatures.
In order to withstand the high operating temperatures, some parts of the furnace are made from refractory metals such as molybdenum, tungsten, titanium and zirconium. These refractory metals are mechanically and physically resistant to dimensional changes under the extreme temperature conditions of an operating glass melting furnace. Electrodes and other parts of a glass melting furnace can be made from these refractory metals. A refractory metal wall can be employed to prevent contamination of the glass by deterioration of the refractory insulating brick wall.
The corrosive effects of molten glass are well known. At the high temperatures present in a glass melting furnace the glass attacks most materials, causing spalling and other forms of deterioration. Molybdenum and tungsten, however, can withstand the corrosive effects of molten glass, and titanium and zirconium can be protected from the corrosive effects of molten glass.
During start up, however, the typical refractory metal experiences an environment which causes considerable deterioration. Typically, a glass furnace is started up by heating to a molten state a small amount of glass batch in the vicinity of a few of the electrodes. The power is then slowly increased to the electrodes and gradually the temperature of the furnace, and the amount of the molten glass in the furnace is increased. During this time, the electrode is heated to its nearly operational temperature of about 3000.degree. F., and yet is partially exposed to the air in the furnace, rather than being covered up with molten glass. This subjects the molybdenum or other refractory metal electrode, or other furnace part, to extremely rapid deterioration due to rapid oxidation.
Several methods have been developed in the prior art for avoiding deterioration of the refractory metal parts because of contact with the atmosphere. One of these is to provide either an inert gas, such as nitrogen or argon, or a reducing gas to provide a protective envelope around the reducing gas to provide a protective envelope around the electrode, the reducing gas being one such as methane or hydrogen. Another method is to try to heat the glass with temporary electrodes and/or products of combustion while keeping the operating refractory metal electrode cool during the furnace heating period. For example, a water-cooled jacket could be placed around the electrode during the time in which the molten glass is built up around the electrode. Then, when the protective water-cooled jacket is removed, the electrode will experience only molten glass, and will not be exposed to the atmosphere.
All of these methods are somewhat undesirable in that they either require extra equipment in the way of additional start up electrodes, or in that they require somewhat complicated procedures, such as the removal of the protective water-cooled jacket.
The idea of applying a protective coating to the electrode itself to ward off deterioration during start up has been tried: a protective coating of molybdenum disilicide has been employed, but has been found to provide only a few days of protection, whereas about 7 days of protection is required for furnace start up. Thus, there is a need for an improved way to protect refractory metal parts in a furnace for melting materials at high temperatures during the start up operation.